Oscillator circuit



March 24, 1936. w, F. COTTER ET AL 2,034,974

OSCILLATOR CIRCUIT Filed June 12, 1934 IN ENTORS Mum/7 607775? f MAR/0NE. BOA .0

ATTORNEY Patented Mar. 24, 1936 ears QFFEQE.

OSCILLATOR CIRCUIT William F. Cotter and field, Mass,

assignors to Marion E. Bond, Spring- United American Bosch Corporation,Springfield, Mass, a corporation of New York Application June 12,

6 Claims.

This invention relates to tuned oscillator circuits and moreparticularly to oscillator circuits which may be tuned to differentfrequency bands.

It is desirable to have superheterodyne receivers which are adapted toreceive a total frequency range much greater than that included in thepresent broadcast band. These receivers have an intermediate amplifierwhich operates at a fixed frequency and the function of the oscillatoris to beat with any received frequency and produce the resultantfrequency for which the intermediate amplifier was designed. Theoscillator circuit may be tuned to a frequency equal to a given signalfrequency plus or minus the intermediate frequency, and the properintermediate beat frequency will result. In single control receivershaving ganged tuning condensers on a single shaft, an auxiliarycombination of parallel and series capacities must be added to theoscillator section in order to maintain the proper frequencydifferential between the oscillator output and the incoming signal overthe entire tuning range of the oscillator condenser. The values of theseadditional capacities are a function of the oscillator coverage ratio,which is obtained by dividing the high frequency end of a given tuningrange by the low frequency end. In the broadcast'band, which usuallyterminates in the lowest frequency to which the receiver can be tuned,the oscillator coverage ratio is the least. The parallel or oscillatortrim condenser therefore needs to be larger, and conversely the seriesor lag condenser needs to be smaller, than those employed in the higherfrequency bands, in order to have the oscillator track at the properfrequency differential with the pre-selector tuning condensers. Thenearer in ratio the intermediate frequency is to the lowest carrierfrequency of a given band, the more important these values become. Theexplanation of circuits to be made later will disclose an improved meansof altering the values of these additional capacities in accordance withthe change of tuning bands. In the higher frequency tuning ranges, theenergy output of the oscillator drops under the usual type of circuitdesign, whereas it should always bear a definite ratio with the energyof the signal in the modulator tube. We propose an auxiliary means ofenergy transfer between elements of the oscillator circuit which willmaintain the oscillator output more constant under a wide range offrequency variation.

The primary object of this invention is, therefore, to provide animproved oscillator circuit which is more efiicient.

1934, Serial No. 730,180

Another object is to provide an oscillator having a more constant energyoutput over a wide frequency range.

A further object is to provide an oscillator which will maintain a moreaccurate frequency differential with the preselector tuning elements.

Other objects and advantages will in part be obvious and in part be setforth in the accompanying specification and claims when read inconnection with the drawing showing the improved oscillator circuitdiagrammatically.

The numeral I indicates a vacuum tube oscillator, having a plate 2, acontrol grid 3, and a cathode 4. The customary cathode heater element isnot shown since it is not connected in any manner to the variouscircuits and elements comprising this invention. The plate 2 isconnected to a suitable source of potential through a. resistance 5.Between the resistance 5 and the plate is connected one side of ablocking condenser g 6, the other side of which leads to a platefeedback inductance l. A lead runs from one side of a high frequency lagcondenser 8 to inductance l, and the other side of the condenser isgrounded. From the grid 3, a connection goes to a condenser 9, and alead l8 joins the opposite condenser ter-- minal to a tuned gridcircuit. A grid inductance H is included in the circuit, and ismagnetically coupled (M) with the feedback inductance l. A

low frequency lag condenser i2 is connected bei tween the end ofinductance ii and ground. A main grid tuning condenser i3, having a highfrequency trim condenser M shunted across it, is

in the circuit between ground and grid lead it.

A high frequency circuit which serially includes 1:

inductance l5, resistance l6, and the low frequency trim condenser i1,is connected in parallel with the grid inductance ll between lead it]and ground. A frequency changing switch i8 is connected between theungrounded side of the high frequency lag condenser 8 and. the lower endof the high frequency inductance IS. The grid 3 and cathode i of theoscillator tube are joined by a resistance i 9.

The operation is as follows. When the switch I8 is open, the oscillatoroperates on the lower frequency band and oscillation is maintained inthe circuit including inductance I, the magnetic coupling (M),inductance H and the high frequency lag condenser 8. Feedback ismaintained through the coupling (M) between inductances l and H. Themain tuning condenser I3 is shunted by an effective trim capacitycomprising condensers i4 and H plus any stray capacity to ground ofinductance I5, and the effective oscillator tracking or lag condenser isI2. The resistance l6 serves to damp out any secondary oscillations ofhigher frequency due to inductance l being also in the circuit. Thereactance of inductance l5, resistance l6, and condenser ll isessentially capacitive and does no harm on the lower frequency band.

When the switch I8 is closed the system operates on the higher frequencyband. The effective trim capacity is condenser I4, the effective trackor lag condenser is 8. The resistance 16 and condenser I! are in effectabsorbed in the much greater capacity of the high frequency lagcondenser 8. Oscillation is now maintained by the feedback of thevoltage appearing across condenser 8, which is common to the plate andgrid circuit, assisting the relatively weak magnetic coupling betweenthe inductances l and E5. The high frequency inductance I5 is of suchlow value that the effect of inductance I l becomes negligible in thetuning of the circuit, and very little current flows through it.

It will be seen then, that oscillation feedback is maintained in thelower frequency band, only by means of magnetic coupling. On the higherband it is maintained through a combination of magnetic and capacityfeedback, resulting from the voltage drop across the high frequency lagcondenser B.

It will be apparent that many changes and modifications may be made byanyone skilled in the art without departing from the true spirit andscope of the invention as expressed in the following claims.

Having described the invention, what is claimed 1. In an oscillatorcircuit for operation over a plurality of frequency bands, an oscillatortube, a plate and a grid circuit connected to said tube, a platefeedback inductance and a plurality of parallel connected gridinductances of different values, means for establishing a magneticcoupling between said plate inductance and said grid inductances, a bandchanging switch connected to the smaller of said grid inductances, acapacity connected in series with said switch, and means including saidcapacity for establishing a capacitive feedback in assistance to saidmagnetic feedback when said switch is closed.

2. In an oscillator tube circuit for operation over a plurality offrequency bands, a, plate and a grid circuit connected to said tube, aplate feedback inductance and a plurality of parallel connected gridinductances of different values, means for establishing a magneticfeedback coupling between said plate inductance and said gridinductances, and means in series with one of said grid inductances foradding a capacitive feedback to said magnetic feedback when operating ata frequency higher than those comprising the lowest of said frequencybands.

3. In an oscillator tube circuit for operation over a plurality offrequency bands, a plate and a grid circuit connected to said tube, aplate feedback inductance, a grid inductance for operation in onefrequency band, a smaller grid inductance connected in parallel withsaid first named grid inductance for operation in a higher frequencyband, means for establishing a magnetic feedback coupling between saidplate inductance and said grid inductances, and means in series withsaid smaller grid inductance for adding a capacitive feedback to saidmagnetic feedback between said plate inductance and' said smaller gridinductance.

4. In an oscillator tube circuit for operation over a plurality offrequency bands, a plate and a grid circuit connected to said tube, aplate feedback inductance, a grid inductance for operation in onefrequency band, a smaller inductance in parallel with said gridinductance for operation in a higher frequency band, means foreffectively including any one of said grid inductances in said gridcircuit, means for establishing a magnetic feedback coupling betweensaid plate inductance and said grid inductances, and means for adding acapacitive feedback to said magnetic feedback between said plateinductance and said smaller grid inductance.

5. In an oscillator tube circuit for operating over a plurality offrequency bands, a plate and a grid circuit connected to said tube, aplate feedback inductance and a plurality of parallel connected gridinductances of different values, means for establishing a magneticfeedback coupling between said plate inductance and said gridinductances, a resistance in series connection with one of said gridinductances, and a band changing switch connected in parallel with saidresistance and in series with said last named grid inductance.

6. In an oscillator tube circuit for operating over a plurality offrequency bands, a plate and a, grid circuit connected to said tube, aplate feedback inductance and a plurality of parallel connected gridinductances of different values, means for establishing a magneticfeedback coupling between said plate inductance and said gridinductances, a resistance in series connection with one of said gridinductances, a band changing switch connected in parallel with saidresistance and in series with said last named grid inductance, and meanscontrolled by said switch for adding a capacitive feedback to saidmagnetic feedback when operating at a frequency higher than thoseincluded in the lowest of said frequency bands.

WILLIAM F. COTTER. MARION E. BOND.

